How high can the CO2 concentration in the air go? It is a bit like looking at the stock market except that the CO2 does not go down. For the first time in human history, concentrations of the greenhouse gas carbon dioxide (CO2) could rise above 400 parts per million (ppm) for sustained lengths of time throughout much of the Northern Hemisphere as soon as May 2013. To provide a resource for understanding the implications of rising CO2 levels, Scripps Institution of Oceanography at UC San Diego is providing daily updates of the Keeling Curve, the record of atmospheric CO2 measured at Hawaii’s Mauna Loa. These iconic measurements, begun by Charles David (Dave) Keeling, a world-leading authority on atmospheric greenhouse gas accumulation and Scripps climate science pioneer, comprise the longest continuous record of CO2 in the world, starting from 316 ppm in March 1958 and approaching 400 ppm today with a familiar saw-tooth pattern. For the past 800,000 years, CO2 levels never exceeded 300 parts per million.

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Many scientists credit Keeling's original graph with first bringing the world's attention to the current increase of carbon dioxide in the atmosphere. Prior to Keeling, the concentration of carbon dioxide in the atmosphere was thought to be affected by constant variability. Keeling had perfected the measurement techniques and observed strong diurnal behavior with steady values of about 310 ppm in the afternoon at three locations: (Big Sur near Monterey, the rain forests of Olympic Peninsula and high mountain forests in Arizona). By measuring the ratio of two isotopes of carbon, Keeling attributed the diurnal change to respiration from local plants and soils, with afternoon values representative of the free atmosphere. By 1960, Keeling and his group had determined that the measurement records from California, Antarctica, and Hawaii were long enough to see not just the diurnal and seasonal variations, but also a year-on-year increase that roughly matched the amount of fossil fuels burned per year.

"I wish it weren’t true, but it looks like the world is going to blow through the 400-ppm level without losing a beat," said Scripps geophysicist Ralph Keeling, who has taken over the Keeling Curve measurement from his late father. "At this pace we’ll hit 450 ppm within a few decades."

The website keelingcurve.ucsd.edu offers background information about how CO2 is measured, the history of the Keeling Curve, and resources from other organizations on the current state of climate. An accompanying Twitter feed, @keeling_curve, also provides followers with the most recent Keeling Curve CO2 reading in a daily tweet.

Dave Keeling began recording CO2 data at Mauna Loa and other locations after developing an ultraprecise measurement device known as a manometer. Ralph Keeling took over the program in 2005 and also heads a program at Scripps to measure changes in atmospheric oxygen. The Scripps O2 and CO2 programs make measurements of CO2 and other gases at remote locations around the world, including Antarctica, Tasmania, and northern Alaska. The Scripps programs are complementary to many other programs now measuring CO2 and other greenhouse gases worldwide.

Scientists estimate that the last time CO2 was as high as 400 ppm was probably the Pliocene epoch, between 3.2 million and 5 million years ago, when Earth’s climate was much warmer than today. CO2 was around 280 ppm before the Industrial Revolution, when humans first began releasing large amounts of CO2 to the atmosphere by the burning of fossil fuels. By the time Dave Keeling began measurements in 1958, CO2 had already risen from 280 to 316 ppm.

Various proxy measurements have been used to attempt to determine atmospheric carbon dioxide levels millions of years in the past. While these measurements give much less precise estimates of carbon dioxide concentration than ice cores, there is evidence for very high CO2 volume concentrations between 200 and 150 Ma of over 3,000 ppm and between 600 and 400 Ma of over 6,000 ppm. In more recent times, atmospheric CO2 concentration continued to fall after about 60 Ma. About 34 Ma, the time of the Eocene—Oligocene extinction event and when the Antarctic ice sheet started to take its current form, CO2 is found to have been about 760 ppm, and there is geochemical evidence that volume concentrations were less than 300 ppm by about 20 Ma.

Each year, the concentration of CO2 at Mauna Loa rises and falls in a sawtooth fashion, with the next year higher than the year before. The peak of the sawtooth typically comes in May. If CO2 levels don’t top 400 ppm in May 2013, they almost certainly will next year, Keeling said.